Semiconductor device having fuse and its manufacture method

ABSTRACT

A semiconductor device has: a fuse having one end applied with a first voltage, and a MOS transistor having source, gate and drain and a connection point between the other end of the fuse and one of the source and drain, a second voltage lower than the first voltage applied to the other of the source and drain, wherein: the first and second voltages, characteristics of the MOS transistor and a resistance of the fuse are selected so that the fuse can be broken down when a predetermined program voltage is applied to the gate; and the resistance of the fuse is set to such a value as a voltage difference between a voltage at the connection point and the second voltage is lower than a drain voltage of the MOS transistor at which a drain current starts saturating, when the program voltage is applied to the gate.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on Japanese Patent Application No.2001-340872, filed on Nov. 6, 2001, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] A) Field of the Invention

[0003] The present invention relates to a semiconductor device withfuses and its manufacture method, and more particularly to techniquesregarding fuse elements used in a trimming circuit or a redundancycircuit of a semiconductor integrated circuit.

[0004] B) Description of the Related Art

[0005] A trimming circuit and a redundancy circuit are often formed in asemiconductor integrated circuit. If a fuse circuit having fuse elementsis used as a trimming circuit or a redundancy circuit, a trimmingprocess and the like can be performed during or after the manufacture ofa semiconductor integrated circuit so that the characteristics of thecircuit can be improved as much as possible.

[0006] Japanese Patent Laid-open Publication HEI-7-307389 discloses inFIG. 1 a circuit having a plurality of parallel connections of a serialconnection of a fuse element and a MOS transistor. It discloses that acurrent drive ability necessary for obtaining a breakdown current forbreaking down a fuse element is given by a function of the gate width Wof a selection transistor:

I _(D) =μC _(ox)(W/L)×(1/2)×(V _(GS) −V _(Y))²

[0007] where I_(D) is a drain current of a selection transistor in asaturation region, and μ is a mobility of carriers. C_(ox) is a gatecapacitance of the selection transistor, W is a gate width and L is agate length. V_(CS) is a gate-source voltage and V_(Y) is a thresholdvoltage.

[0008] If the value I_(D) of a saturation drain current necessary forbreaking down a fuse element is known, the gate width W (size) of thetransistor capable of breaking down the fuse element can be estimatedfrom the above-described equation. This analysis adopts the assumptionthat the saturation current of a MOS transistor is used for breakingdown a fuse.

[0009] In order to melt and break down a fuse element, it is necessaryto flow current through the fuse element and heat it to a temperatureover the melting point thereof. For example, if single crystal siliconor polysilicon is used as the material of a fuse element, a relativelylarge current is required because the melting point of silicon is ashigh as about 1420° C. It is therefore necessary to make large the sizeof a selection transistor, which hinders high integration of deviceelements. According to the above-described Publication, a bipolartransistor having a high current drive ability is used as a selectiontransistor to obtain a large current.

[0010] Most of recent integrated circuits are MOS type ICs using MOSFETs as fundamental device elements. If a bipolar transistor is requiredto be formed in a MOS type IC, the element structure becomes complicatedand additional processes are necessary.

SUMMARY OF THE INVENTION

[0011] An object of this invention is to reduce an area occupied by afuse circuit having a fuse element and a selection transistor andfabricated in a MOS IC, by using a MOSFET as the selection transistorand reducing the area occupied by the selection transistor.

[0012] According to one aspect of the present invention, there isprovided a semiconductor device comprising: a fuse element capable ofbeing electrically broken down by flowing current therethrough, a firstvoltage being applied to one end of the fuse element; and a MOS typetransistor having source, gate and drain terminals and a connectionpoint between the other end of the fuse element and one of the sourceand drain terminals, a second voltage lower than the first voltage beingapplied to the other of the source and drain terminals, wherein: thefirst and second voltages, characteristics of the MOS type transistorand a resistance value of the fuse element are selected so that the fuseelement can be broken down when a predetermined program voltage isapplied to the gate terminal; and the resistance value of the fuseelement is set to such a value as a voltage difference between a voltageat the connection point and the second voltage is lower than a drainvoltage of the MOS type transistor at which a drain current startssaturating, when the program voltage is applied to the gate terminal

[0013] According to another aspect of the present invention, there isprovided a semiconductor device comprising: a fuse element capable ofbeing electrically broken down by flowing current therethrough, a firstvoltage being applied to one end of the fuse element; and a MOS typetransistor having source, gate and drain terminals and a connectionpoint between the other end of the fuse element and one of the sourceand drain terminals, a second voltage lower than the first voltage beingapplied to the other of the source and drain terminals, wherein: thefirst and second voltages, characteristics of the MOS type transistorand a resistance value of the fuse element are selected so that the fuseelement can be broken down when a predetermined program voltage isapplied to the gate terminal; and the resistance value of the fuseelement is further set to such a value as a minimum power capable ofbreaking down the fuse element is not smaller than 90% of a maximumconsumption power of the fuse element calculated from current-voltagecharacteristics of the MOS type transistor.

[0014] According to another aspect of the present invention, there isprovided a semiconductor device comprising: a fuse element capable ofbeing electrically broken down by flowing current therethrough, a firstvoltage being applied to one end of the fuse element; and a MOS typetransistor having source, gate and drain terminals and a connectionpoint between the other end of the fuse element and one of the sourceand drain terminals, a second voltage lower than the first voltage beingapplied to the other of the source and drain terminals, wherein: thefirst and second voltages, characteristics of the MOS type transistorand a resistance value of the fuse element are selected so that the fuseelement can be broken down when a predetermined program voltage isapplied to the gate terminal; and the resistance value of the fuseelement is further set to such a value as a breakdown current of thefuse element is in a range from 80% to 98% of a saturation drain currentof the MOS type transistor.

[0015] A power supplied to the semiconductor device can be usedefficiently for breaking down the fuse element.

[0016] According to a further aspect of the present invention, there isprovided a method of manufacturing a semiconductor device, comprisingsteps of: forming on a substrate a serial connection of a fuse elementand a MOS type transistor, the fuse element being capable of beingelectrically broken down by flowing current therethrough, and the MOStype transistor having source, gate and drain terminals and a connectionpoint between one end of the fuse element and one of the source anddrain terminals; and applying a voltage higher than a drain voltage ofthe MOS type transistor at which a drain current starts saturation,between another end of the fuse element and the other of the source anddrain terminals, applying a predetermined program voltage to the gateterminal, and breaking down the fuse element by setting a voltage at theconnection point between the fuse element and the MOS type transistor toa voltage lower than a drain voltage of the MOS type transistor in asaturation region in which a drain current saturates.

[0017] According to another aspect of the present invention, there isprovided a method of manufacturing a semiconductor device, comprisingsteps of: forming on a substrate a serial connection of a fuse elementand a MOS type transistor, the fuse element being capable of beingelectrically broken down by flowing current therethrough, a firstvoltage being applied to one end of the fuse element, the MOS typetransistor having source, gate and drain terminals and a connectionpoint between one end of the fuse element and one of the source anddrain terminals, and a second voltage lower than the first voltage beingapplied to the other of the source and drain terminals; and applying avoltage higher than a drain voltage of the MOS type transistor at whicha drain current starts saturation, between the other end of the fuseelement and the other of the source and drain terminals, applying apredetermined program voltage to the gate terminal, and breaking downthe fuse element by setting a voltage at the connection point of the MOStype transistor in a voltage range in which a consumption power of thefuse element is not smaller than 90% of a maximum consumption power ofthe fuse element calculated from current-voltage characteristics of theMOS type transistor.

[0018] According to another aspect of the present invention, there isprovided a method of manufacturing a semiconductor device, comprisingsteps of: forming on a substrate a serial connection of a fuse elementand a MOS type transistor, the fuse element being capable of beingelectrically broken down by flowing current therethrough, a firstvoltage being applied to one end of the fuse element, the MOS typetransistor having source, gate and drain terminals and a connectionpoint between one end of the fuse element and one of the source anddrain terminals, and a second voltage lower than the first voltage beingapplied to the other of the source and drain terminals; and applying avoltage higher than a drain voltage of the MOS type transistor at whicha drain current starts saturation, between the other end of the fuseelement and the other of the source and drain terminals, applying apredetermined program voltage to the gate terminal, and breaking downthe fuse element by setting a voltage at the connection point betweenthe fuse element and the MOS type transistor in a voltage range in which80% to 98% of a saturation current of the MOS type transistor flows.

[0019] As above, in the fuse circuit made of a serial connection of afuse element and a selection transistor, the power supplied to the fusecircuit can be used efficiently for breaking down the fuse element. Itis therefore possible to reduce the area occupied by the selectiontransistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a circuit diagram of a fuse circuit having a fuseelement and a MOSFET as a selection transistor for the fuse element

[0021]FIG. 2A is a graph showing the typical current-voltagecharacteristics of a MOSFET to be used as a selection transistor.

[0022]FIG. 2B is a graph showing time change of the voltage at theinterconnection point between a selection transistor and a fuse.

[0023]FIG. 3 is a graph showing a source-drain voltage dependency of afuse consumption power

[0024]FIG. 4 is a plan view of a semiconductor device according to anembodiment of the invention.

[0025]FIG. 5 is a cross sectional view of the semiconductor device takenalong line V-V′ shown in FIG. 4.

[0026]FIG. 6 is a graph showing the current-voltage characteristics of asemiconductor device of an embodiment, the graph showing a change in anoperating point of a selection transistor relative to a gate voltage.

[0027]FIG. 7 is a graph showing the current-voltage characteristics of asemiconductor device of an embodiment, the graph showing the resistanceof a fuse element changed to make an operating point enter in atransition region.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] In this embodiment, the term “fuse element” is intended to meanan element which can be broken down when a current at least equal to apredetermined value is flowed. The term “selection transistor” isintended to mean a transistor connected in series with a fuse element,determines whether current is to be flowed through the fuse element andif to be flowed, determines the amount of current.

[0029] Prior to describing the embodiment of the invention, theprinciple of the invention will be described with reference to FIGS. 1to 3.

[0030]FIG. 1 is a circuit diagram of a fuse circuit having a fuseelement and an n-channel MOS type field effect transistor (MOSFET) usedas a selection transistor for the fuse element. FIG. 2A is a graphshowing the typical current-voltage characteristics of a MOSFET used asa selection transistor. FIG. 2B is a graph showing time change of thevoltage at the interconnection point 7 a between the selectiontransistor 3 and the fuse 1. FIG. 3 is a graph showing a powerconsumption of a fuse element relative to the source-drain voltage of aMOS type field effect transistor.

[0031] As shown in FIG. 1, a fuse circuit A has a fuse element 1 and aselection transistor 3 made of a MOS type FET serially connected to thefuse element.

[0032] One end 1 a of the fuse element 1 is connected, for example, to apower supply voltage V_(DD). The other end 1 b of the fuse element isconnected to the drain terminal 5 a of the selection transistor 3. Thesource terminal 5 b of the selection transistor 3 is connected to theground (GND).

[0033] As shown in FIG. 2A, the drain current-voltage characteristics ofthe selection transistor 3 have a linear build-up region 10 a and asaturation region 10 b. In the linear region 10 a, the drain voltageV_(DS) is low and as the drain voltage V_(DS) is raised, the draincurrent I_(D) increases almost linearly. In the saturation region 10 b,the drain (to source) voltage V_(DS) is high and generally a constantdrain current flows independently of the drain voltage V_(DS). Inpractical cases, in the saturation region, as the drain voltage israised, the drain current gradually increases in some case. Also in thiscase, the region where the drain current changes almost linearly withthe drain voltage, at a rate significantly lower than that in thebuild-up region 10 a, is called the saturation region. Between thelinear region and saturation region, a region exists in which anincrement of the drain current is not proportional to an increment ofthe drain voltage. This region is called a transition region 10 c. It ispractically difficult to strictly distinguish between the transitionregion, linear region and saturation region. It is therefore definedthat one end of the transition region is a current point 20% lower thanthe linear characteristics of the saturation region and the other endthereof is a current point 2% lower than the linear characteristics ofthe saturation region.

[0034] The current-voltage characteristics of the fuse element 1 aregenerally the linear characteristics that current and voltage areproportional. Therefore, the voltage at a connection point (node) 7between the fuse element 1 and selection transistor 3 corresponds to thedrain voltage (voltage at the connection point 7, in this specification,it is represented by V_(DSO)) at a connection point (operating point) onthe current-voltage characteristics between the selection transistor 3and fuse element 1.

[0035] As shown in FIG. 2A, a voltage V_(F) applied across the fuseelement 1 is equal to (V_(DD)-V_(DSO)). A voltage V_(TR) applied betweenthe source and drain of the selection transistor 3 is equal to D_(DSO).

[0036] A consumption power P_(T) of the selection transistor 3 and aconsumption power P_(F) of the fuse element 1 are given by the followingequations (1) and (2):

P _(T) =V _(DSO) ×I _(D1)  (1)

P _(F)=(V _(DD) −V _(DSO))×I _(D1)  (2)

[0037] where I_(D1) is a current flowing through a serial connection ofthe fuse element 1 and selection transistor 3 when a predeterminedprogram voltage V_(p) is applied to the gate terminal 5 c of theselection transistor 3.

[0038] It was found that the resistance of a polysilicon fuse mayincrease in the course of breaking down the fuse. In FIG. 2A, thischange is shown by the shift of the load curve from LC1 to LC1′. Theload curve LC1 represents the state just after the selection transistoris turned on, and the load curve LC1′ represents the state just beforethe fuse is broken down. The source-drain voltage is decreased fromV_(DSO) to V_(DSO)′. The drain current is decreased from I_(D1) toI_(D1)′. Then the power consumption in the fuse becomes

P _(F)′=(V _(DD) −V _(DSO)′)×I_(D1)′  (3).

[0039]FIG. 2B shows an example of time change of the voltage at theinterconnection between the selection transistor 3 and the fuse 1, inthe circuit of FIG. 1. When the gate voltage Vp is 0 V, the selectiontransistor is turned off and the voltage at the interconnection terminal7 a is at the source voltage V_(DD)(=5 V). When the gate voltage Vp israised to 5 V, the selection transistor 3 is turned on to allow acurrent I_(D1) to flow through the fuse 1. The voltage at theinterconnection 7 a is decreased to V_(DSO) by the voltage drop acrossthe fuse 1. At this state, the resistance of the fuse can be representedas Rf, and the voltage V_(DSO) can be expressed as

V _(DSO) =V _(DD)−(Rf×I _(D1))  (4).

[0040] Along with the lapse of time in which a current is allowed toflow through the fuse, the voltage at the interconnection showsgenerally slow decrease with minor and irregular deviations. Thisrepresents the general resistance increase of the fuse.

[0041] When a breaking or cutting current is flown through a fuse, thepower consumption in-the fuse will generate heat, and the temperature ofthe fuse will be elevated by the generated heat. Along with thetemperature increase, the grains in the fuse may grow or change, and thegrain boundaries may be molten. The increase of the fuse resistance maybe ascribed to such phenomena.

[0042] Then, the voltage at the interconnection shows a rapid decreaseto about 0 V. This represents that the resistance of the fuse becomesinfinite, i.e. the fuse is broken down. The graph shows some huntingvibration after the voltage rapidly decreased to zero, which may be dueto the rapid voltage change at the interconnection and not be the resultof the current change through the fuse.

[0043] The point just before the rapid decrease will be called“immediately before the break-down of the fuse”. At this point, thedrain current is I_(D1)′, the voltage at the interconnection isV_(DSO)′, and the resistance of the fuse is Rf′. Then,

V _(DSO) ′=V _(DD)−(Rf′×I _(D1)′)  (5).

[0044] The voltage at the interconnection immediately before thebreakdown of the fuse is lower than the voltage just after the selectiontransistor is turned on, V_(DSO)′<V_(DSO), which means that a highervoltage is applied across the fuse. Also, I_(D1)′<I_(D1), which meansthat the current through the fuse is decreased, and Rf′>Rf, which meansthat the resistance of the fuse is increased.

[0045] In the typical MOS transistor characteristics, the power supplyvoltage V_(DD) was set to 5 V and the program voltage V_(p) applied tothe gate terminal 5 c of the selection transistor 3 was set 5 V, and theconsumption power of the fuse element 1 was calculated by using theequation (2) or (3) by changing the resistance of the fuse element 1.

[0046] In this specification, the drain voltage at which the saturationregion of the selection transistor starts, i.e., the voltage at whichthe drain current starts taking a nearly constant value (or the draincurrent increases approximately linearly with the drain voltage at arate significantly lower than that in the build-up region 10 a) iscalled a saturation voltage. More specifically, in the linearcharacteristics in the saturation region, a drain voltage at which thecurrent value increases to 98% of the linear characteristics is called asaturation voltage.

[0047]FIG. 3 is a graph plotting the relation between a consumptionpower P_(F) of a fuse element and a source-drain voltage. In FIG. 3, thedrain current-fuse element. As the resistance of a fuse element ischanged, V_(DS) and I_(D1) change.

[0048] As shown in FIG. 3, as the source-drain voltage V_(DS) of theselection transistor rises, the consumption power of the fuse elementincreases and takes a maximum value near at V_(DS)=1.5 V. The draincurrent is outside of the linear region and in the transition region. Asthe source/drain voltage V_(DS) exceeds 1.5 V, an increase in the draincurrent becomes small, and because of a lowered voltage applied acrossthe fuse element, the consumption power of the fuse element graduallylowers. As the drain current enters the saturation region, the fuseconsumption power reduces generally linearly.

[0049] A general fuse element is set so that its operating point is inthe saturation region of the selection transistor, e.g., at about 3 V.Therefore, of the power consumed by the fuse circuit, the power consumedby the selection transistor is more than about a half of the totalsupply power. Therefore, the ratio of the power consumed by the fuseelement to break the fuse element is made small.

[0050] Based upon the above-described theoretical and experimentalstudies, the inventor has noticed that the operating point of the fusecircuit is better set not in the saturation region of the selectiontransistor but in the boundary region between the saturation region andlinear region, i.e., in the transition region. By setting the operationpoint in the transition region, of the total consumption power of thefuse circuit, the ratio of the power consumed by the fuse element can bemade large. In other words, the ratio of the power loss in the selectiontransistor can be made small.

[0051] Even if the load curve just after the selection transistor isturned on crosses the transition region of the source-drain I-Vcharacteristics, if the load curve immediately before the breakdown ofthe fuse crosses the linear build-up region of the source-drain I-Vcharacteristics, the available power becomes small as can be seen fromFIG. 3. It may results in a failure of breaking down a fuse. Thus, it ispreferable that the resistance value of the fuse element and thecharacteristics of the selection transistor is so selected that the loadcurve immediately before the break-down of the fuse crosses thetransition region of the source-drain I-V characteristics.

[0052] Referring to FIG. 2A, the load curve LC1′ is preferably selectedto cross the transition region 10 c of the I-V characteristics of theselection transistor, for effectively and stably breaking down a fuse.

[0053] In connection with these studies, a semiconductor deviceaccording to an embodiment of the invention will be described withreference to FIGS. 4 and 5.

[0054]FIG. 4 is a plan view of the semiconductor device according to theembodiment. FIG. 5 is a cross sectional view of the semiconductor devicetaken along line V-V′ shown in FIG. 4. The semiconductor device shown inFIGS. 4 and 5 shows the specific structure of the fuse circuit shown inFIG. 1. The manufacture processes for a fuse circuit will be described.

[0055] As shown in FIGS. 4 and 5, an isolation region 2 a, 2 b is formedin a predetermined area of a p-type well (impurity concentration: 10¹⁶to 10¹⁷ cm⁻³) of a semiconductor substrate 11 by local oxidation ofsilicon (LOCOS). The isolation region may be formed by shallow trenchisolation (STI) instead of LOCOS. The isolation region 2 a, 2 b definesactive regions where transistors are formed. Ions are implanted into thesurface layer of the active region to slightly increase a p-typeimpurity concentration to adjust a threshold voltage.

[0056] A gate insulating film 15 a of silicon oxide is formed on thesurface of the active region, for example, by thermal oxidation. A gateelectrode 17, for example, of polycide (lamination ofsilicide/polysilicon), is formed on the gate insulating film 15 a.Polysilicon is doped with n-type impurities of about 10²⁰ cm⁻³. Theconcept of polycide is intended to include salicide. The gate electrodemay be made of only polysilicon.

[0057] At the same time when the gate electrode 17 is formed, a polycidelayer (or polysilicon layer) 23 used as a fuse element is formed on theisolation region 2 a.

[0058] Side spacer insulating films 15 b may be formed on the side wallsof the gate electrode 17. In this case, the side spacers are also formedon the side walls of the fuse element 23. Prior to forming the sidespacers, ion implantation for LDD (lightly doped drain) is performed toform LDD regions having an n-type impurity concentration of 10¹⁷ to 10¹⁸cm⁻³.

[0059] After the side spacers are formed, n-type impurities areimplanted in the semiconductor substrate regions on both sides of thegate electrode 17 at a high impurity concentration (10²⁰ to 10²¹ cm⁻³).Source/drain regions 5 a/5 b are therefore formed in the semiconductorsubstrate regions on both sides of the gate electrode, and impuritiesare doped also in the gate electrode 17 and fuse element 23 so that theresistances thereof are lowered.

[0060] An interlayer insulating film 21, for example, of silicon oxide,is formed over the semiconductor substrate, covering the gate electrode17 and polycide resistance layer 23. Openings 18 a and 18 b are formedthrough the interlayer insulating film 21, reaching the source/drainregions 5 a/5 b on both sides of the gate electrode 17, and openings 25and 27 are also formed reaching the upper surfaces of opposites ends ofthe polycide layer 23.

[0061] A first wiring layer 31 a is formed which contacts the uppersurface of one end of the fuse layer 23 via the opening 25. At the sametime, a second wiring layer 31 b is formed which contacts the uppersurface of the other end of the fuse layer 23 via the opening 27 andcontacts the source/drain region 5 a via the opening 18 a. Further, athird wiring layer 31 c is formed which contacts the source/drain region5 b via the opening 18 b.

[0062] As shown in FIG. 4, a read terminal 7 a for reading stored datais formed which is connected to the fuse element 1 and selectiontransistor 3 branched from the second wiring layer 31 b. Similarly, afifth wiring layer 7 b is formed which extends from the gate terminal 5c and constitutes an input terminal to which a program voltage forbreaking down the fuse element 1 is applied. A terminal 7 c for applyingthe power supply voltage VDD to one end of the fuse element 1 and aterminal 7 d for applying a ground potential to the source/drain region5 b are also formed.

[0063] With the above processes, the fuse circuit having the fuseelement 1 and the selection transistor 3 of MOSFET can be formed.

[0064] The characteristics of the fuse circuit will be described withreference to FIGS. 6 and 7. FIG. 6 is a graph showing thecurrent-voltage characteristics of the fuse circuit when the gatevoltage V_(g) of the selection transistor is changed. FIG. 7 is a graphshowing the current-voltage characteristics of the fuse circuit. Thepower supply voltage is represented by V_(DD). A program voltage appliedto the gate terminal of the selection transistor to break the fuseelement is represented by V_(p). A line L indicates the current-voltagecharacteristics of a fuse element at the fuse resistance of Rf.

[0065] As shown in FIG. 6, as the gate voltage Vg to be applied to thegate terminal of a selection transistor is raised from Vg1 to Vg2 and toVg3, the drain current I_(D) of the selection transistor increases. Thecross point between the I-V characteristics of the selection transistorand the I-V characteristics of the fuse element also change from P1 toP2 and to P3. A difference voltage between the power supply voltageV_(DD) and the drain voltage at the operating point P is a voltageapplied across the fuse element. Therefore, as the gate voltage Vg israised, the consumption power of the fuse element increases. Theoperating point P3 is used because a drain current sufficient forbreaking down the fuse element can be obtained. The resistance value ofthe fuse element is selected so that the fuse element can be broken downin the state that the operating point P3 is in the transition region R3between the linear region R1 and saturation region R2.

[0066] The operation near at the operating point P3 will be described indetail with reference to FIG. 7.

[0067] The drain current-voltage characteristics are represented by L1in the state that a program voltage Vp is applied to the gate electrodeof a selection transistor. The fuse circuit constituted of the selectiontransistor having the characteristics L1 and the fuse element has anoperating point P3 (node between the fuse element and selectiontransistor) in the transition region R3 between the linear region R1 andsaturation region R2. The source/drain voltage of the selectiontransistor at the operating point P3 is represented by Vm and the draincurrent at the operating point P3 is represented by Im. The draincurrent Im is equal to the current flowing through the fuse element. Theresistance value of the fuse element is represented by Rf.

[0068] If the operating point is in the transition region R3, the powerloss by the selection transistor can be made small. In order to set theoperating point in the transition region, it is sufficient that the fusehas a resistance value in the range between the resistance valuescalculated from lines L2 and L3. The line L2 represents thecurrent-voltage characteristics of the fuse element having such aresistance value as the operating point P3 is located at the highestvoltage side in the transition region R3, and the line L3 represents thecurrent-voltage characteristics of the fuse element having such aresistance value as the operating point P3 is located at the lowestvoltage side in the transition region R3.

[0069] It is preferable to set the resistance value of the fuse elementto such a value as a difference voltage between a second voltage (sourcevoltage, in this example, ground potential) and the voltage at the node(connection point) between the fuse element and selection transistor islower than the drain voltage at which the drain current of a MOStransistor starts saturating under the condition that a program voltageis applied to the gate terminal of the selection transistor. In thiscase, the ratio of invalid voltage not contributing to breaking down thefuse element can be lowered.

[0070] It is preferable to set the resistance value of the fuse elementto such a value as a difference voltage between the second voltage andthe voltage at the connection point between the fuse element andselection transistor is higher than the drain voltage in the linearregion where the drain current of a MOS transistor is proportional tothe drain voltage under the condition that a program voltage is appliedto the gate terminal of the selection transistor. In this case, thedrive ability of the MOS transistor can be utilized sufficiently and thesize of the transistor can be made as small as necessary. The areaoccupied by the transistor can therefore be reduced.

[0071] The resistance value of a fuse element is preferably set to sucha value that the minimum power capable of breaking down the fuse elementis not smaller than 90% of the maximum consumption power of the fuseelement calculated from the current-voltage characteristics of a MOStransistor. The transistor characteristics have generally a variation of10%. It is preferable to have this margin of 10% in order that asmallest transistor can reliably flow an optimum current.

[0072] It is preferable that the resistance value of a fuse element isset so that the breakdown current of the fuse element calculated fromthe fuse current-voltage characteristics is in the range from 80% to 98%of the saturation current of a MOS transistor. In this case, the powersufficient for breaking down the fuse element can be retained even ifthere is some manufacture variation.

[0073] The drain current-voltage characteristics of the selectiontransistor indicated by the line L1 are the characteristics at theprogram voltage Vp applied to the gate terminal of the selectiontransistor. Generally, the program voltage is a voltage (first voltage:power supply voltage) applied to the fuse circuit in order tosufficiently turn on the selection transistor.

[0074] The program voltage may be set slightly smaller than the powersupply voltage by considering a voltage drop across the transistor. Onthe other hand, the program voltage may be set slightly higher than thepower supply voltage (first voltage) in order to make the selectiontransistor enter an on-state of a sufficiently low resistance. Theseprogram voltages are called “approximately equal to” the first voltage.

[0075] With the above-described settings, the consumption power of theselection transistor can be reduced and the power supplied to the fusecircuit can be used efficiently effectively to break the fuse element.The gate width of a selection transistor necessary for breaking down afuse element can be narrowed so that the area occupied by the fusecircuit can be made small.

[0076] After a fuse element and a selection transistor are seriallyconnected, the fuse element is broken down in accordance with any one ofthe following setting methods.

[0077] A first setting method will be described.

[0078] A first voltage is applied between opposite ends of a serialconnection of a fuse element and a selection transistor, the firstvoltage being higher than a drain voltage at which a drain current ofthe selection transistor starts saturating. A predetermined programvoltage is applied to the gate electrode of the selection transistor.The fuse element is broken down under the condition that a voltage atthe connection point between the fuse element and selection transistoris higher than the drain voltage in the build-up linear region of theselection transistor and lower than the drain voltage in the saturationregion.

[0079] A second setting method will be described.

[0080] A first voltage is applied between opposite ends of a serialconnection of a fuse element and a selection transistor, the firstvoltage being higher than a drain voltage at which a drain current ofthe selection transistor starts saturating. A predetermined programvoltage is applied to the gate electrode of the selection transistor. Inthis case, the program voltage is set to such a value as the consumptionpower of the fuse element calculated from the fuse elementcharacteristics becomes not smaller than 90% of the maximum consumptionpower obtained from the calculated current-voltage characteristics ofthe fuse element. With these settings, the fuse element is broken down.

[0081] A third setting method will be described.

[0082] A first voltage is applied between opposite ends of a serialconnection of a fuse element and a selection transistor, the firstvoltage being slightly higher than a drain voltage at which a draincurrent of the selection transistor starts saturating. A predeterminedprogram voltage is applied to the gate electrode of the selectiontransistor. In this case, the program voltage is set to such a value asthe voltage at the connection point between the fuse element andselection transistor falls in the voltage range allowing to flow 80% to98% of the saturation drain current of the selection transistor. Withthese settings, the fuse element is broken down.

[0083] The characteristics of a selection transistor and a fuse elementchange with environments, particularly a temperature change. If theremay be a temperature change, it is necessary to design the selectiontransistor and fuse element by sufficiently considering a change in thecharacteristics to be caused by the temperature change.

[0084] The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It is apparent to those skilled in the art that variousmodifications, improvements, combinations, and the like can be made.

What we claim are:
 1. A semiconductor device comprising: a fuse elementcapable of being electrically broken down by flowing currenttherethrough, a first voltage being applied to one end of said fuseelement; and a MOS type transistor having source, gate and drainterminals and a connection point between the other end of said fuseelement and one of the source and drain terminals, a second voltagelower than the first voltage being applied to the other of the sourceand drain terminals, wherein: the first and second voltages,characteristics of said MOS type transistor and a resistance value ofsaid fuse element are selected so that said fuse element can be brokendown when a predetermined program voltage is applied to the gateterminal; and the resistance value of said fuse element is set to such avalue as a voltage difference between a voltage at the connection pointand the second voltage is lower than a drain voltage of said MOS typetransistor at which a drain current starts saturating, when the programvoltage is applied to the gate terminal.
 2. A semiconductor deviceaccording to claim i, wherein the resistance value of said fuse elementis further set to such a value as a voltage difference between a voltageat the connection point and the second voltage is higher than the drainvoltage in a linear region where the drain current of said MOS typetransistor is proportional to the drain current, when the programvoltage is applied to the gate terminal.
 3. A semiconductor deviceaccording to claim 1, wherein the program voltage of said MOS typetransistor is approximately equal to the first voltage.
 4. Asemiconductor device according to claim 1, wherein the gate terminal ofsaid MOS type transistor and said fuse element are made of a same layer.5. A semiconductor device according to claim 1, wherein the same layeris made of polycide.
 6. A semiconductor device comprising: a fuseelement capable of being electrically broken down by flowing currenttherethrough, a first voltage being applied to one end of said fuseelement; and a MOS type transistor having source, gate and drainterminals and a connection point between the other end of said fuseelement and one of the source and drain terminals, a second voltagelower than the first voltage being applied to the other of the sourceand drain terminals, wherein: the first and second voltages,characteristics of said MOS type transistor and a resistance value ofsaid fuse element are selected so that said fuse element can be brokendown when a predetermined program voltage is applied to the gateterminal; and the resistance value of said fuse element is set to such avalue as a minimum power capable of breaking down said fuse element isnot smaller than 90% of a maximum consumption power of said fuse elementcalculated from current-voltage characteristics of said MOS typetransistor.
 7. A semiconductor device according to claim 6, wherein thegate terminal of said MOS type transistor and said fuse element are madeof a same layer.
 8. A semiconductor device according to claim 7, whereinthe same layer is made of polycide.
 9. A semiconductor devicecomprising: a fuse element capable of being electrically broken down byflowing current therethrough, a first voltage being applied to one endof said fuse element; and a MOS type transistor having source, gate anddrain terminals and a connection point between the other end of saidfuse element and one of the source and drain terminals, a second voltagelower than the first voltage being applied to the other of the sourceand drain terminals, wherein: the first and second voltages,characteristics of said MOS type transistor and a resistance value ofsaid fuse element are selected so that said fuse element can be brokendown when a predetermined program voltage is applied to the gateterminal; and the resistance value of said fuse element is set to such avalue as a breakdown current of said fuse element is in a range from 80%to 98% of a saturation drain current of said MOS type transistor.
 10. Asemiconductor device according to claim 9, wherein the gate terminal ofsaid MOS type transistor and said fuse element are made of a same layer.11. A semiconductor device according to claim 10, wherein the same layeris made of polycide.
 12. A method of manufacturing a semiconductordevice, comprising the steps of: forming on a substrate a serialconnection of a fuse element and a MOS type transistor, the fuse elementbeing capable of being electrically broken down by flowing currenttherethrough, and the MOS type transistor having source, gate and drainterminals and a connection point between one end of the fuse element andone of the source and drain terminals; and applying a voltage higherthan a drain voltage of the MOS type transistor at which a drain currentstarts saturation, between another end of the fuse element and the otherof the source and drain terminals, applying a predetermined programvoltage to the gate terminal, and breaking down the fuse element bysetting a voltage at the connection point between the fuse element andthe MOS type transistor to a voltage lower than a drain voltage of theMOS type transistor in a saturation region in which a drain currentsaturates.
 13. A method of manufacturing a semiconductor device,comprising the steps of: forming on a substrate a serial connection of afuse element and a MOS type transistor, the fuse element being capableof being electrically broken down by flowing current therethrough, afirst voltage being applied to one end of the fuse element, the MOS typetransistor having source, gate and drain terminals and a connectionpoint between one end of the fuse element and one of the source anddrain terminals, and a second voltage lower than the first voltage beingapplied to the other of the source and drain terminals; and applying avoltage higher than a drain voltage of the MOS type transistor at whicha drain current starts saturation, between the other end of the fuseelement and the other of the source and drain terminals, applying apredetermined program voltage to the gate terminal, and breaking downthe fuse element by setting a voltage at the connection point of the MOStype transistor in a voltage range in which a consumption power of thefuse element is not smaller than 90% of a maximum consumption power ofthe fuse element calculated from current-voltage characteristics of theMOS type transistor.
 14. A method of manufacturing a semiconductordevice, comprising the steps of: forming on a substrate a serialconnection of a fuse element and a MOS type transistor, the fuse elementbeing capable of being electrically broken down by flowing currenttherethrough, a first voltage being applied to one end of the fuseelement, the MOS type transistor having source, gate and drain terminalsand a connection point between one end of the fuse element and one ofthe source and drain terminals, and a second voltage lower than thefirst voltage being applied to the other of the source and drainterminals; and applying a voltage higher than a drain voltage of the MOStype transistor at which a drain current starts saturation, between theother end of the fuse element and the other of the source and drainterminals, applying a predetermined program voltage to the gateterminal, and breaking down the fuse element by setting a voltage at theconnection point between the fuse element and the MOS type transistor ina voltage range in which 80% to 98% of a saturation current of the MOStype transistor flows.
 15. A semiconductor device according to claim 1,wherein said resistance value of said fuse element is a valueimmediately before the fuse element is broken down.
 16. A semiconductordevice according to claim 6, wherein said resistance value of said fuseelement is a value immediately before the fuse element is broken down.17. A semiconductor device according to claim 9, wherein said resistancevalue of said fuse element is a value immediately before the fuseelement is broken down.
 18. A method of manufacturing a semiconductordevice according to claim 12, wherein said resistance value of said fuseelement is a value immediately before the fuse element is broken down.19. A method of manufacturing a semiconductor device according to claim13, wherein said resistance value of said fuse element is a valueimmediately before the fuse element is broken down.
 20. A method ofmanufacturing a semiconductor device according to claim 14, wherein saidresistance value of said fuse element is a value immediately before thefuse element is broken down.